FIG. 1 illustrates a switched power supply 100 that may be used to provide power to a computer system. Switched power supply 100 includes a power supply 102, a transformer 104, a diode 106, a capacitor 110, and a field effect transistor (FET) 112. Transformer 104 in turn includes a primary coil 114 and a secondary coil 116. The capacitor 110 operates as an output filter. The switched power supply 100 shown in FIG. 1 may be a portion of a so-called fly back converter that provides power to a computer system.
Switched power supplies of computer systems are monitored to insure that a proper amount of current is provided to the computer systems. Switched power supplies are monitored by measuring the current flowing through the primary coil. The current flow through the primary coil can be monitored by monitoring the current flow IL through FET 112. IL can be measured via the voltage VD at the drain of FET 112. More particularly, IL can be measured in accordance with the following equation:
IL=VD/RDSONxe2x80x83xe2x80x83(1)
where RDSON represents the source to drain resistance of FET 112 when FET 112 is in the on state. IL can be compared against predetermined current values to determine whether IL is operating in an acceptable range. For example, IL may be compared to IM where IM represents a maximum limit of the acceptable range of current flowing through FET 112.
The value of RDSON in equation 1 above can be calculated as follows:
RDSON=RDSON(25)xc2x7(1+AT)xe2x80x83xe2x80x83(2)
where RDSON(25) represents the resistance of FET 112 between the drain and source at 25xc2x0 C. when FET 112 operates in the on state, A is a well-known temperature coefficient of RDSON, and T is the temperature measured in centigrade of FET 112 operating in the on state at the time voltage VS is compared with the voltage VM. Using equation (2), equation (1) can be translated into:
IL=VS/(RDSON(25)xc2x7(1+AT))xe2x80x83xe2x80x83(3)
Several problems exist with the prior art method of monitoring current via equation (3) above. The first problem is that the temperature T of FET 112 is difficult to measure. A thermocouple for generating a signal indicative of temperature, could be attached to FET 112, and the output of the thermocouple could be input into a circuit that generates IL as a function of the temperature output of the thermocouple, a calculated value for RDSON(25), and VS in accordance with equation (3) above. Attaching a thermocouple to FET 112 will be expensive and would give rise to reliability issues. Alternatively, T could be presumed. In other words, a presumption could be made that FET 112 will operate in the on state at a predetermined temperature TP. Under this presumption, IL could be generated as a function of:
IL=VS/((RDSON(25)xc2x7(1+ATP)).xe2x80x83xe2x80x83(4)
If the presumption for TP is inaccurate, comparing IL to IM may not be a reliable means of determining whether current flowing through FET 112 is operating below a predetermined maximum.
The second problem with equation (3) above relates to differences between the actual and calculated values of RDSON(25). The actual value of RDSON(25) is subject to a statistical distribution. In practice, RDSON(25) varies from FET to FET due to fabrication variances. For example, one FET fabricated on a first wafer may have an RDSON(25) which differs from that of another FET fabricated on a different part of the wafer or on another wafer. The variances may be due to, for example, variances in doping density. The accuracy of equation (3) is dependent upon how close the actual RDSON(25) value is to the calculated value of RDSON(25). If the calculated and actual values of RDSON(25) differ significantly, than comparing IL to IM may not be a reliable means of determining whether current flowing through FET 112 is operating below a predetermined maximum.
The temperature dependency of RDSON could be up to 30 to 40% over the span of ambient temperature to max operating temperature. The statistical distribution of RDSON(25) due to fabrication variances could be as large as plus or minus 30%. Accordingly, the model above may not lead to an accurate monitoring of current provided by switched power supply 100
Disclosed is a method and apparatus for FET current sensing using the voltage drop across the drain to source resistance that eliminates dependencies on temperature of the FET and/or statistical distribution of the initial value of drain to source resistance of the FET. In one embodiment, first and second FETs are provided. Each of the first and second FETs include a gate, a source, and a drain. The gate of the first FET is configured to receive a first voltage, and the source of the first FET is configured to be coupled to ground. The gate of the second FET is configured to receive a second voltage, and the source of the second FET is configured to be coupled to ground. A circuit is also provided and includes first and second input nodes coupled to the drains of the first and second FETs, respectively. The circuit is configured to generate a signal as a function of a voltage measured at the drain of the first FET with respect to ground, wherein the signal is proportional to a current flowing into or out of the drain of the first FET.
In one embodiment, the first and second FETs are formed adjacent to each other on a semiconductor wafer ensuring close matching of their electrical characteristics so that the first and second FETs operate in the on state at substantially the same temperature.
In one embodiment, the signal generated by the circuit is proportional to a ratio of substrate areas over which the first and second FETs are respectively formed.